Recommendation I.210 describes the principles for defining telecommunication services supported by an ISDN including the concept of bearer services,teleservices and supplementary services.It also provides the means for the definition and description of such services.The purpose of this Recommendation is to define a recommended set of supplementary services to be used in association with basic bearer services and with basic teleservices.These definitions form the basis for detailed descriptions as given in Recommendations I.251 to I.257,which are used to define the network capabilities required.